Song Qin-Hua, Tang Wen-Jian, Ji Xue-Bao, Wang Hong-Bo, Guo Qing-Xiang
Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, P. R. China.
Chemistry. 2007;13(27):7762-70. doi: 10.1002/chem.200700251.
cis-syn Cyclobutane pyrimidine dimers, major UV-induced DNA lesions, are efficiently repaired by DNA photolyases. The key step of the repair reaction is a light-driven electron transfer from the FADH(-) cofactor to the dimer; the resulting radical anion splits spontaneously. Whether the splitting reaction requires considerable activation energy is still under dispute. Recent reports show that the splitting reaction of a dimer radical anion has a significant activation barrier (0.45 eV), and so photolyases have to provide considerable energy. However, these results contradict observations that cis-syn dimer radical anions split into monomers at -196 degrees C, and that the full process of DNA photoreactivation was fast (1.5-2 ns). To investigate the activation energies of dimer radical anions, three model compounds 1-3 were prepared. These include a covalently linked cyclobutane thymine dimer and a tryptophan residue (1) or a flavin unit (3), and the covalently linked uracil dimer and tryptophan (2). Their properties of photosensitised splitting of the dimer units by tryptophan or flavin unit were investigated over a large temperature range, -196 to 70 degrees C. The activation energies were obtained from the temperature dependency of splitting reactions for 1 and 2, 1.9 kJ mol(-1) and 0.9 kJ mol(-1) for the thymine and uracil dimer radical anions, respectively. These values are much lower than that obtained for E. coli photolyase (0.45 eV), and are surmountable at -196 degrees C. The activation energies provide support for previous observations that repair efficiencies for uracil dimers are higher than thymine dimers, both in enzymatic and model systems. The mechanisms of highly efficient enzymatic DNA repair are discussed.
顺式 - 顺式环丁烷嘧啶二聚体是主要的紫外线诱导的DNA损伤，可被DNA光解酶有效修复。修复反应的关键步骤是光驱动的电子从FADH(-)辅因子转移到二聚体；产生的自由基阴离子会自发分裂。二聚体自由基阴离子的分裂反应是否需要相当大的活化能仍存在争议。最近的报道表明，二聚体自由基阴离子的分裂反应具有显著的活化能垒（0.45电子伏特），因此光解酶必须提供相当多的能量。然而，这些结果与以下观察结果相矛盾：顺式 - 顺式二聚体自由基阴离子在-196℃时分裂成单体，并且DNA光复活的整个过程很快（1.5 - 2纳秒）。为了研究二聚体自由基阴离子的活化能，制备了三种模型化合物1 - 3。这些化合物包括共价连接的环丁烷胸腺嘧啶二聚体和色氨酸残基（1）或黄素单元（3），以及共价连接的尿嘧啶二聚体和色氨酸（2）。在-196至70℃的大温度范围内研究了它们通过色氨酸或黄素单元对二聚体单元进行光敏分裂的性质。通过1和2的分裂反应的温度依赖性获得活化能，胸腺嘧啶和尿嘧啶二聚体自由基阴离子的活化能分别为1.9 kJ mol(-1)和0.9 kJ mol(-1)。这些值远低于大肠杆菌光解酶的活化能（0.45电子伏特），并且在-196℃时是可以克服的。这些活化能为先前的观察结果提供了支持，即在酶促和模型系统中，尿嘧啶二聚体的修复效率高于胸腺嘧啶二聚体。本文还讨论了高效酶促DNA修复的机制。
